It has been a long time since I played vanilla, but I don't ever remember enemies able to occupy the same space as the player. This is happening quite often to me using prboom+ on skill 5, e.g. http://youtu.be/vsGFOwgqJoI. It is very frustrating, especially when trying to perform a difficult jump like in this example ;-P

It's a boom map (Vanguard) so my complevel options are limited. Even further limited to complevel 11 (MBF) because of the ledge behavior issue covered in another thread.

No, in vanilla a monster can't teleport if you're standing on his spawn spot. Not sure what option is causing this behavior in that port, I don't use it.

Edit: I see the monster is respawning in Nightmare difficulty? In that case it wouldn't surprise me if monsters could respawn on top of the player in vanilla in that case, since the feature was added later.

You can try -complevel 7 - i.e. Boom's "Doom compatibility" mode. This means that Boom specials will work. However, it enforces Doom-like behaviour for certain things that Boom handled differently (such as stair building and floor motion), which means that some maps simply won't work at all. Most Boom maps were tested with full Boom behaviour, after all.

Vorpal said:Is this a known issue? Or did vanilla simply behave this way also?

Are you using complevel 11? If so, there is a bug at that complevel introduced by Lee Killough's recoded clipping behavior for MBF (monsters can respawn and resurrect on top of stuff). A compatibility option was later added with PrBoom that disables MBF's buggy clipping (comp_moveblock), but unfortunately, the complevels that acknowledge it also suffer from the ledge compatibility bug.

You can try using the default complevel and setting comp_moveblock to 1 in your cfg. You won't be able to disable Boom's ledge behavior, unfortunately, but it may solve the clipping issue you're experiencing.